A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
For lithographic processing, the location of patterns in subsequent layers on the substrate should be as precise as possible for a correct definition of device features on the substrate, which features all should have sizes within specified tolerances. The overlay error (i.e., the mismatch between subsequent layers) should be within well-defined tolerances for creating functional devices.
To this end, an overlay measurement module is generally used for determining the overlay error of a pattern on the substrate with a mask pattern as defined in a resist layer on top of the pattern.
The overlay measurement module typically performs the measurement with optics. The position of the mask pattern in the resist layer relative to the position of the pattern on the substrate is determined by measuring an optical response from an optical marker which is illuminated by an optical source. The signal generated by the optical marker is measured by a sensor arrangement. Using the output of the sensors the overlay error can be derived. Typically, the patterns on which overlay error are measured are located within a scribe lane in between target portions.
Two basic concepts are known for overlay metrology.
A first concept relates to measurement of overlay error that is image based. A position of an image of the pattern on the substrate is compared to the position of the mask pattern in the resist layer. From the comparison the overlay error is determined. An example to measure overlay error is the so-called box-in-box structure, in which the position of an inner box within an outer box is measured relative to the position of the outer box.
Image based overlay error measurement may be sensitive to vibrations and also to the quality of focus during measurement. For that reason, image based overlay error measurement may be less accurate in environments that are subjected to vibrations, such as within a track system. Also, image-based overlay measurements may be susceptible to aberrations in the optics that may further reduce the accuracy of the measurement.
A second concept relates to measurement of overlay error that is diffraction based. In the pattern layer on the substrate a first grating is located, and in the resist layer a second grating is located with a pitch that is, substantially identical to the first grating. The second grating is located nominally on top of the first grating. By measuring the intensity of the diffraction pattern as generated by the first and second grating superimposed on each other, a measure for the overlay error may be obtained. If some overlay error is present between the first and second grating, this is detectable from the diffraction pattern.
In diffraction based overlay error measurement, only the first and second gratings may be illuminated, since light that reflects from adjacent regions around the gratings interferes with the intensity level of the diffraction pattern. However, a trend emerges to have overlay error measurements close to critical structures within a die (and not necessarily within the scribe lane). Also, there is a demand to reduce the size of gratings so as to have a larger area available for circuitry. To some extent, such demands can be accommodated by a reduction of the cross section of the illumination beam that impinges on the first and second gratings so as to avoid illumination of the region outside the gratings. However, the minimal cross-section of the illumination beam is fundamentally limited by the laws of physics (i.e. limitation due to diffraction). Below, the cross-sectional size in which diffraction of the beam occurs will be referred as the diffraction limit.